A colleague and I are collaborating on study of plant diversity within agro-ecosystems of the Eastern Ghats, India. One of our main study objectives is compare species richness of several habitat types which include both forested and non-forested habitats. Obviously in addition to mean richness per sampling unit (in our case, intercept along 10-m line transects) we are interested in both a) how many new species are found with additional samples, and b) controlling for sample size (no. of transects), what is the expected number of species (rarefaction). Here is where the confusion arises. When comparing richness of forested and non-forested habitats by sample based accumulation curves, the two habitat types are quite similar. Yet, when comparing the habitat types by individual-based curves, clearly the rate of new species sampled in forests is much greater. Hence, an initial interpretation of these data is that near equivalent richness between these habitat types is a largely a function of abundance (with greater abundance being in the non-forested habitat). A similar example would be comparing richness in three forest categories (say for example, young, mid-age, and old) with equivalent sample sizes in each. But lets say the experimenter decided to spend twice as long in one of the three treatments, hence over biasing the number of individuals sampled (although sample sizes are the same). To make valid comparisons, would one not then be in situation of needing to use individual-based rather than sample-based rarefaction?
Any thoughts are much appreciated. Brian D. Campbell Department of Biology Queens University Kingston, Ontario. Canada.
